Chipped the stucco off my porch post today and the base was basically sponge

I ran into something similar a few years back when replacing the base support for a small covered patio. It wasn't a post, but the main beam that was anchored into the concrete footing, and the rot was actualy happening right where the beam met the sill plate. We had poured a standard mix but the wood was still absorbing water from the ground. The original builder just stuffed it in and covered it with cheap masonry.

What ended up working for us was moving the connection entirely. Instead of trying to seal the wood to the concrete, we decided to use a steel post base with a heavy-duty collar. I went with a galvanized one because we were worried about rust, and that allowed the beam to sit entirely inside a protective sleeve rather than touching the damp foundation itself. It made the installation a little trickier because of the required cut angles, but it eliminated the moisture problem.

Another thing I learned is that the wood type matters. While pressure-treated is fine, if you want maximum longevity in a splash zone, you realy want something like cedar or redwood for the exposed ends, then treat the rest. The contrast in color helps you see if water is starting to bead up again.

I dealt with a similar issue a few years ago when fixing some ledger board supports for a shed. It wasn't a massive structural failure like what you described, but the bottom section of a 4x4 post had soft spots, mostly near the grade line. I managed to stabilize it before it got worse. Instead of going straight to replacement, I used a structural epoxy consolidation kit because the decay was localized to maybe six or seven inches of the original piece. The plan was to grind out all the soft, spongy wood and then use the epoxy to penetrate the remaining sound wood. This basicly froze the moisture in the remaining fibers and prevented further breakdown. It's definately more involved than just swapping the whole thing out, and you have to be extremely careful when cutting into anything that might be tied into the main structure. You want to make sure you are cutting on a clean plane and that the new wood is properly secured, even if it is temporarily braced first. For future reference, if the rot is only localized like that, consolidation might save you the cost of a whole new pressure-treated piece and all the associated labor.

Same story at my place but it was the porch column on a covered entry, not buried in veneer. The fix that held up for me was a standoff base bolted to the slab with a stand-off height, so the end grain sits a couple inches above the concrete and any water just runs off. I also sloped the top of my footing slightly away from the post, seemed to help. One thing I'd add on your downspout: get a splash block or extension on it too, because I rerouted mine and the water still bounced back off the gravel untill I added one. And check the other posts while you're at it, mine had a twin on the opposite corner that was just as bad and I never noticed until later.